Register a new access-type with IANA

NOTE: I am writing this myself: https://github.com/Pandapip1/multiaddr-access-type

All contributions are welcome and will be acknowledged!

## Summary

I suggest that a new access type be created for multiaddrs, as per [RFC 1341](https://datatracker.ietf.org/doc/html/rfc1341) section 7.3.3

## Example

```
Content-type: message/external-body; access-type=multiaddr; multiaddr="/p2p/zb2rhe5P4gXftAwvA4eXQ5HJwsER2owDyS9sKaQRRVQPn93bA"
```

## Motivation

- This turns IPFS into a DIY peer-to-peer CDN, with the additional advantage that recently-accessed websites are served by the client's IPFS node.
- This allows web servers to tell clients to request files off of IPFS or any other protocol supported by multiaddrs, without the web server being able to access it
- [Useful for blockchain-based web servers](https://eips.ethereum.org/EIPS/eip-5219) that can't access IPFS directly
- Also reduces latency by reducing the size of round trips
- Without this: Server requests large file, receives large file, sends large file to client, client receives large file
- With this: Server tells client to request large file, client can request and receive large file (and bonus: provide a progress indicator)